(a) Scope of the Invention
This invention involves a system for measuring glaze ice by means of a microprocessor, with a new release mechanism incorporated therewith. More specifically, the invention relates to a glaze-ice detector that eliminates the measurement errors of a system presently in use. Obviously, the expression glaze-ice also includes rime formations.
(b) Description of Prior Art
In an organization specializing in the transmission of electricity, as is the case with the Applicant, the network for observing freezing-rain storms is very important for tower design and the real-time remote monitoring of existing lines.
A study of the weather in a proposed line corridor comprises two equally important parts: the intensity of the impact on the lines during a freezing-rain storm, and the frequency of such an event or its probability in time. Using the information collected with glaze-ice detection equipment, lines can be designed with a good knowledge of the risks, and the extent and duration of mechanical failure during a severe accumulation of glaze-ice or rime can be predicted. The observations and information obtained from a network of glaze-ice detectors enables the transmission line project engineer to design a line at a level of risk that is highly acceptable from an economic point of view.
An electrical utility's high-voltage lines often pass through mountainous zones that are subjected to severe weather conditions. Certain line sections are more exposed than others, and on some occasions, rime and glaze-ice have been the direct cause of major collapses. In many other occasions, rime and glaze-ice have caused the damaging or rupture of overhead spacer-dampers, ground-wires, conductor, etc., which produce or may produce major power failures.
Because of the remoteness of the Applicant's transmission lines in isolated areas, and in view of the meteorological conditions associated with storms, it is very difficult to observe rime or glaze-ice. And when freezing-rain is falling, it is difficult to find out the amplitude, duration and gravity of the storm as well as its effect on the behaviour of the lines.
Ideally, this physical link should have sufficient mechanical reliability to ensure the transmission of power even when atmospheric conditions are unfavorable. However, experience in previous years has enabled utilities to identify certain sectors where the lines are more vulnerable. Although a fool-proof line does not exist, several solutions have been proposed to solve the problem. Except for moving the route of the lines, these solutions (reinforcement, or a strategy of rapid reconstruction to limit the duration of the line outage) do not eliminate the risk of a fault due to glaze-ice, and it is largely for this reason that we have designed a glaze-ice real-time observation network that can give the alarm and indicate the seriousness of the situation when freezing-rain occurs.
There are few instruments capable of detecting and/or measuring glaze-ice. And the instruments that are available on the market have been developed mainly for aviation needs.
The instrument that was best suited to the Applicant's needs and the most compatible with its teletransmission system was the ice detector of Rosemount Instruments Ltd., hereinafter called the "Rosemount". This ice detector is described in Canadian Patent No. 808,827 issued on Mar. 18, 1969 to the Rosemount Engineering Company Limited as well as in U.S. Pat. Nos. 3,277,459 of Oct. 4, 1966 and 3,341,835 of Sept. 12, 1967, both issued to the Rosemount Engineering Company.
The Applicant has used about thirty of these devices since 1972, within the framework of its glaze-ice observation programs for the existing or future 735-kV transmission system. This has enabled it to acquire good experience with this type of apparatus.
To summarize the essentials of the mode of operation of the Rosemount glaze-ice detector: it comprises a detector, a heater and a holding tube that ensures the required distance between the detector and the mounting plate. The assembly is screwed to a casing containing the electronic portion.
This apparatus detects the presence of glaze-ice by means of the detector which consists of an ultrasonic rod that vibrates axially. The natural frequency of this rod decreases as glaze-ice accumulates.
When the ice thickness reaches 0.51 mm (0.02 inches), a signal is emitted by the device's electronic interface and a de-icing system begins to operate. After seven (7) seconds, the accumulation of ice has been melted by an assembly of heating elements inside the holding tube, the captor cools down and the instrument is ready to receive a new accumulation of glaze-ice or rime.
As the Rosemount device was developed for the needs of aviation, it does not completely meet the needs of the Applicant. The Rosemount device consumes too much electricity. The voltage of 115 volts AC used for heating the detector is too high if the device is utilized in an isolated region. Finally, the Rosemount apparatus often gives erroneous results during freezing-rain storms, if it is attached to a fixed support. Under these conditions, water droplets have been seen at the base of the detector, after heating. These droplets are eventually transformed into ice after the heating period ends. This presence of ice at the base of the detector produces false measurements and false alarms, making the device ineffectual because of the unreliability of the information obtained.
The frozen water droplets at the base of the detector cause basic oscillation frequency to increase, as opposed to a decrease when glaze-ice accumulates. Since the Rosemount apparatus utilizes an analog comparison method that does not take into consideration the polarity of the variations in the basic frequency, the results obtained are consequently erroneous under the conditions just described. The positive variation in the basic frequency will hereinafter be referred to as frequency inversion.
There is therefore a real need for a device accuracy and without error, while consuming only a small quantity of electrical energy at low voltage.